The invention pertains to circuit for decoding traffic information message tone signals whose frequency, the message tone frequency, is the information marking a traffic information message. The tone signals are broadcast as amplitude-modulated signals which are received and demodulated with a conventional radio receiver.
In the journal "Funkschau", 1974, pages 535 to 538, a system for broadcasting traffic information to radio listeners is described which was introduced in Germany at that time and which is now used in other countries, also. In this system, a message tone signal is transmitted during a traffic information broadcast. In addition, regional tone signals are transmitted. These identifying signals are low-frequency signals which are impressed on the carrier, having a frequency of 57 KHz in the known system, by amplitude modulation. They are derived from the carrier by integral frequency division.
As stated in the journal "Rundfunktechnische Mitteilungen" 1974, pp. 193 to 202, where this traffic information broadcasting system is described in detail, the system parameters were so chosen that the decoder circuits required for automatic traffic information reception were compatible with conventional analog-signal-processing receiver circuits and, particularly, did not interfere with one another. The hitherto used decoder circuits are, therefore, analog circuits as well.